26-4 As the early universe expanded and cooled, most of the matter and antimatter annihilated each other

As soon as the flood of matter and antimatter appeared in the universe, collisions between particles and antiparticles began to produce numerous high-energy gamma rays. As these gamma rays collided, they promptly turned back into the particles and antiparticles from which they came. As a result, the rate of pair production soon equaled the rate of annihilation. For example, for every electron and positron that annihilated each other to create gamma rays (Figure 26-13b), two gamma rays collided elsewhere to produce an electron and a positron (Figure 26-13a). In other words, annihilation and pair production reactions proceeded with equal vigor, and as many particles and antiparticles were being created as were being destroyed.

As the universe continued to expand, all the gamma-ray photons became increasingly redshifted. As a result, the temperature of the radiation field fell. Due to their frequent interaction, radiation and particles of all kinds were in thermal equilibrium: All particle species, including photons, were at the same temperature. Hence, as the radiation temperature decreased, the temperature of particles of different types decreased as well.

From Quark Confinement to Particle-Antiparticle Annihilation

The first change in the population of particles and antiparticles occurred at t = 10⁻⁶ second, when the temperature was 10¹³ K and particles were colliding with energies of roughly 1 GeV. Prior to this moment, particles collided so violently that individual protons and neutrons could not exist, being constantly fragmented into quarks. After this time, appropriately called the period of quark confinement, quarks were finally able to stick together and became confined within individual protons and neutrons.

As the universe continued to expand, temperatures eventually became so low that the gamma rays no longer had enough energy to create particular kinds of particles and antiparticles. We say that the temperature dropped below the particular particle’s threshold temperature. Collisions between these types of particles and antiparticles continued to add photons to the cosmic-radiation background, but collisions between photons could no longer replenish the supply of particles and antiparticles.

At the same time that quark confinement became possible so that protons and neutrons appeared, the universe also became cooler than the 10¹³-K threshold temperatures of both protons and neutrons. No new protons or neutrons were formed by pair production, but the annihilation of protons by antiprotons and of neutrons by antineutrons continued vigorously everywhere throughout space. This wholesale annihilation dramatically lowered the matter content (particles and antiparticles) of the universe, while simultaneously increasing the radiation (photon) content.

A little later, when the universe was about 1 second old, its temperature fell below 6 × 10⁹ K, the threshold temperature for electrons and positrons. A similar annihilation of pairs of electrons and positrons further decreased the matter content of the universe while raising its radiation content. This radiation field, which fills all space, is the primordial fireball discussed in Section 25-5. This fireball, which dominated the universe for the next 380,000 years, therefore derived much of its energy from the annihilation of particles and antiparticles during the first second after the Big Bang.

Now we have a dilemma. If there had been perfect symmetry between particles and antiparticles, then for every proton there should have been an antiproton. For every electron, there should likewise have been a positron. Consequently, by the time the universe was 1 second old, every particle would have been annihilated by an antiparticle, leaving no matter at all in the universe.

Obviously, a total annihilation of all matter and antimatter never happened. The planets, stars, and galaxies we see in the sky are made of matter, not antimatter. If there were still substantial amounts of antimatter in the universe, it would eventually collide with ordinary matter. We would then see copious amounts of gamma rays being emitted from the entire sky. While we do observe gamma-ray photons from various locations in the universe, they are neither numerous enough nor of the right energy to indicate the presence of much antimatter. Thus, there must have been an excess of matter over antimatter immediately after the Big Bang so that the particles outnumbered the antiparticles.

Quite remarkably, we can estimate the extent of this initial asymmetry between matter and antimatter. In other words, while we essentially see a universe consisting of matter today, we can still infer how much antimatter there was before the antimatter portion of the universe annihilated with most of the matter portion. The key is light: Each annihilation of two particles produces exactly two photons (Figure 26-13b), and after being stretched to longer wavelengths, these are the photons we observe in the cosmic microwave background. As noted in Section 25-5, there are roughly 10⁹ (one billion) photons today in the microwave background for each proton and neutron in the universe. Thus, for every 10⁹ antiprotons, there must have been 10⁹ plus one ordinary protons, leaving one surviving proton after annihilation. Similarly, for every 10⁹ positrons, there must have been 10⁹ plus one ordinary electrons.

While we can infer that the initial matter-antimatter asymmetry was about one additional particle made of regular matter per billion matter-antimatter pairs, we do not understand what process led to this initial asymmetry. There are several ideas for explaining the asymmetry but no hard evidence or agreement on a likely solution, and this initial asymmetry remains one of the great unsolved problems in physics and astronomy.